CENTRES U ET ASSOCIATIONS DE CENTRES COLORÉS
ATOMIC HYDROGEN CENTRES IN IONIC CRYSTALS

Résumé
Cet article est une revue des travaux expérimentaux et théoriques sur les centres de l'hydrogène atomique dans les halogénures alcalins et les fluorures d'alcalins-terreux. En raison de leur structure très simple, ces centres peuvent être considérés comme des modèles pour l'étude des propriétés des défauts ponctuels et leur interprétation théorique par des calculs basés sur les principes fondamentaux.

Abstract
Experimental and theoretical work on atomic hydrogen centres in alkali halides and alkaline earth fiuorides is reviewed. Because of their very simple structure these centres can be regarded as model systems for the study of the properties of point defects and their theoretical understanding by calculations from first principles. From ESR and ENDOR spectroscopy the distribution of the unpaired hydrogen electron over several shells of neighbour ions was determined. The ground state wave function of the centre electron is highly localized. The present theoretical understanding of the structure of the ground state wave function is discussed with emphasis on interstitial centres in the alkali halides. Complete orthogonalisation of the ion wave function and some covalent charge transfer from the halogen ions to the hydrogen atom as well as the consideration of the hydrogen zero point vibrations are important features of the ground state. The results of optical absorption and magneto-optical measurements in the alkali halides are reviewed. The excited state is qualitatively understood to be not of the hydrogen type but to consist of the symmetry allowed linear combinations of the outer halogen p orbitals, whereby interactions between the halogen ions and their spin orbit interaction are to be considered. In alkali chlorides doped with some bromide or iodide interstitial hydrogen centres can be produced in a mixed configuration. Results of recent optical absorption and emission as well as ESR experiments are briefly described.